1. Field of the Invention
The present invention is directed to an optical surface inspection method, more particularly to a method for optical inspection of a defect such as a crack, stain, and irregularity of an article.
2. Description of the Prior Art
There has been a growing demand for optical surface inspection which employs digital image processing for rapid and reliable inspection of a surface defect of an article such as a minute electronic component. In a known inspection system utilizing the digital image processing as disclosed, for example, in the Japanese Early publication No. 62-88946, an image transducer such as a TV camera takes a grey value image of a substantially two-dimensional configuration of an article to be inspected. The grey value image is composed of a number of picture elements or pixels having individual grey values. The grey value of each pixels in the image is compared with a suitable threshold value to provide a binary image which is then analyzed for evaluation of a defect. That is, in this prior image evaluation, a transition between two binary values is judged to be indicative of the defect. However, a serious problem has been encountered in such defect evaluation technique. For instance, it fails to discriminate a defect of which pixel has the grey value on the same side of the threshold value as the grey value of the adjacent pixels of a normal non-defective area. In other words, the threshold value is not effective for recognize such a defect that shows a rather vague border edge line or transition between a defective area and a non-defective area.